Atomic Discovery

Early Models of the AtomEarly Models of the Atom

400 B.C. – Democritus proposed the existence of 400 B.C. – Democritus proposed the existence of fundamental particles of matter that were indivisible and fundamental particles of matter that were indivisible and indestructible - “atomos”.indestructible - “atomos”.

Aristotle thought all matter was continuous; he did not believe in atoms.

Neither idea was supported by any experimental evidence – speculation only.

Foundations of Atomic TheoryFoundations of Atomic Theory

The late 1700’s –definitions and basic laws had The late 1700’s –definitions and basic laws had been discovered and accepted by chemists.been discovered and accepted by chemists.

Element – substance that cannot be broken down by Element – substance that cannot be broken down by ordinary chemical means.ordinary chemical means.

Chemical Reaction – transformation of substance or Chemical Reaction – transformation of substance or substances into one or more new substances.substances into one or more new substances.

Law of Conservation of MassLaw of Conservation of Mass – mass cannot be created or – mass cannot be created or destroyed just changed from one form to another. Mass is destroyed just changed from one form to another. Mass is conserved in reactions. (conserved in reactions. (Antoine Lavosier-Father of Chemistry Antoine Lavosier-Father of Chemistry 1778))1778))

Law of Definite ProportionsLaw of Definite Proportions – a chemical compound contains – a chemical compound contains exactly the same elements in the same proportion regardless of exactly the same elements in the same proportion regardless of sample size. sample size. (Joseph Proust from work of Gay-Lussac & (Joseph Proust from work of Gay-Lussac & Amadeo Avogadro – 1802/1804)Amadeo Avogadro – 1802/1804)

Law of Multiple ProportionsLaw of Multiple Proportions – If two or more different – If two or more different compounds are composed of the same two elements, then the compounds are composed of the same two elements, then the ratio of the masses of those elements will always exist as a ratio ratio of the masses of those elements will always exist as a ratio of small whole numbers. of small whole numbers. (John Dalton - 1808)(John Dalton - 1808)

Dalton’s Atomic Theory 1808Dalton’s Atomic Theory 1808

All elements are composed of tiny indivisible particles All elements are composed of tiny indivisible particles called atoms.called atoms.

Atoms of the same element are identical. The atoms of Atoms of the same element are identical. The atoms of one element are different from the atoms of another one element are different from the atoms of another element.element.

Atoms cannot be subdivided, created or destroyed.Atoms cannot be subdivided, created or destroyed. Atoms combine in simple whole-number ratios.Atoms combine in simple whole-number ratios. Atoms are separated, joined or rearranged in chemical Atoms are separated, joined or rearranged in chemical

reactions. Atoms of one element are never changed into reactions. Atoms of one element are never changed into atoms of another element as a result of a chemical atoms of another element as a result of a chemical reaction.reaction.

Other Contributors to the Atomic Other Contributors to the Atomic TheoryTheory

Amadeo Avogadro-discovered the Amadeo Avogadro-discovered the relationship between volume of gases and relationship between volume of gases and the number of particles in them (1811)the number of particles in them (1811)

Discovery of ElectronsDiscovery of Electrons

1897 – J.J. Thomson – “Cathode Ray Tube Experiment”1897 – J.J. Thomson – “Cathode Ray Tube Experiment”

Showed existence of first know sub-atomic particleShowed existence of first know sub-atomic particle

Determined charge to mass ratio of the electronDetermined charge to mass ratio of the electron

1909 – Robert Millikan found the charge of the electron – 1909 – Robert Millikan found the charge of the electron – “Millikan’s Oil Drop Experiment” “Millikan’s Oil Drop Experiment”

Cathode Ray Tube

High Voltage

Cathode Ray (electrons)

Metal disk (cathode)

Metal disk (anode)

Gas at very low pressure

Cathode Ray Tube

High Voltage

Cathode Ray (electrons)

Metal disk (cathode)

Metal disk (anode)

Gas at very low pressure

Negative plate

Positive plate

Observations from Cathode RayObservations from Cathode Ray

A glow (cathode ray) was formed between A glow (cathode ray) was formed between the cathode and anode.the cathode and anode.

A paddle wheel between the cathode and A paddle wheel between the cathode and anode rolled toward the anode. (had anode rolled toward the anode. (had enough mass)enough mass)

Cathode ray was deflected from a Cathode ray was deflected from a magnetic field.magnetic field.

Cathode ray was deflected by a negative Cathode ray was deflected by a negative charge.charge.

Discovery of Charge of ElectronDiscovery of Charge of Electron

Milikan’s Oil Drop ExperimentMilikan’s Oil Drop Experiment Negative chargeNegative charge Mass of electronMass of electron Concluded electrons are present in all Concluded electrons are present in all

atomsatoms Concluded that atoms are divisible.Concluded that atoms are divisible.

Assumptions based on Discovery Assumptions based on Discovery of Electronof Electron

Because atoms are neutral, there must be Because atoms are neutral, there must be a positive charge to balance the electrons.a positive charge to balance the electrons.

Because electrons have very small mass Because electrons have very small mass compared to an atom, there must be other compared to an atom, there must be other particles in an atom to account for the particles in an atom to account for the other mass.other mass.

Rutherford’s Gold Foil ExperimentRutherford’s Gold Foil Experiment

Rutherford, Geiger & Marsden (1912) -showed that Rutherford, Geiger & Marsden (1912) -showed that most of the atom was empty space, but that atoms most of the atom was empty space, but that atoms had a solid, positive core.had a solid, positive core.

Alpha Particles

Radioactive source

Lead shield

Discovery of ProtonsDiscovery of Protons

1919 -J.J. Thomson & James Chadwick– 1919 -J.J. Thomson & James Chadwick– discovered particles traveling opposite of the discovered particles traveling opposite of the cathode rays.cathode rays.

Determined existence, mass and charge of Determined existence, mass and charge of protonsprotons

Idea had actually been previously proposed by Idea had actually been previously proposed by Goldstein in 1886.Goldstein in 1886.

Cathode Ray Tube

High Voltage

Cathode Ray (electrons)

Metal disk (cathode)

Metal disk (anode)

Gas at very low pressure Negative

plate

Positive plate

protons

NeutronsNeutrons

James Chadwick 1932 - confirmed the existence of the neutron. Neutrons are subatomic James Chadwick 1932 - confirmed the existence of the neutron. Neutrons are subatomic particles with no charge but with a mass nearly equal to that of a proton.particles with no charge but with a mass nearly equal to that of a proton.

Walter Bothe had first reasoned the existence of a third subatomic particle in 1930.Walter Bothe had first reasoned the existence of a third subatomic particle in 1930.

Bothe’s work was based in part on that of Henry Mosely who showed by X-ray analysis that not Bothe’s work was based in part on that of Henry Mosely who showed by X-ray analysis that not all atoms of the same element were identical. (Isotopes – 1907)all atoms of the same element were identical. (Isotopes – 1907)

RadioactivityRadioactivity

Mosely’s X-ray analysis of atoms was an attempt to Mosely’s X-ray analysis of atoms was an attempt to explain radioactivity.explain radioactivity.

1896 – Henri Becquerel – Uranium spontaneously emits 1896 – Henri Becquerel – Uranium spontaneously emits energy.energy.

1898 – Marie & Pierre Curie – first isolated a radioactive 1898 – Marie & Pierre Curie – first isolated a radioactive element - Radiumelement - Radium

““Planetary” Model of the AtomPlanetary” Model of the Atom

Niels Bohr (1913) – developed the “planetary” model of Niels Bohr (1913) – developed the “planetary” model of the atom based upon the following: the atom based upon the following:

Rutherford’s Gold Foil ExperimentRutherford’s Gold Foil Experiment

E = mcE = mc2 2 – Albert Einstein (1905)– Albert Einstein (1905)

Quantum Theory – Max Planck (1910)Quantum Theory – Max Planck (1910)

Properties of Subatomic ParticlesProperties of Subatomic Particles

Particles Symbol Charge Relative Particles Symbol Charge Relative Mass Mass MassMassElectronElectron ee-- 1- 1- 1/1840 amu1/1840 amu 9.11 x 109.11 x 10-28-28 g g

Proton pProton p++ 1+ 1 amu 1+ 1 amu 1.67 x 101.67 x 10-24-24 g g

NeutronNeutron nnºº 0 1 amu 0 1 amu 1.67 x 10 1.67 x 10-24-24 g g

AtomAtom

1010-13 -13 cmcm electronselectrons

protonsprotons neutronsneutrons

1010-8 -8 cmcm

nucleus

Size of the AtomSize of the Atom

Puncher Dome

e-

Aluminum Atom

e-

e-

e-

e-

e- e-

e-

e-

nucleus - size of a marble

1 mm

Outside edge of Al atom

150 me-

goal post

stands